1. Field of the Invention
The present invention relates to a tissue evaluation apparatus and method for rapidly assessing non-invasively the tissue status particularly with respect to the quality of microcirculation, susceptibility to mechanical force related damage and differentiation of bruised or necrotic dermal or sub-dermal tissue from that which is healthy or erythemous.
The apparatus and method of the present invention is particularly effective in, but not limited to, the early detection of a pressure ulcer. Any medical condition or disease which can be assessed by measuring the competence of microcirculatory supply and drainage will also be a potential application for this technique.
2. Discussion of Related Art
The first sign of a pressure ulcer is as an area of persistent redness which may be visible on some skins and not others depending upon their pigmentation. This persistent redness is due to soft tissue being compressed for a long period of time between a bone and a firm surface such as a mattress causing an interruption of the blood supply which the body corrects by means of a temporary elevation of the blood flow to the area.
Clinically, nurses are able to detect early pressure ulcers by the redness of the skin by compression of a reddened area with a finger to xe2x80x98blanchexe2x80x99 the area and assessing the rate at which the blanche disappears. If the reddened area of skin blanches and then returns to red on release of pressure within a predetermined time, this is called blanchable erythema, and is not generally considered to constitute a serious change in skin health by nurses.
In cases where the redness or erythema persists after compression with a finger, called non-blanchable erythema, this is an indication that damage has occurred, due to a more severe interruption of the blood supply and inflammation, either due to excessive pressure or lower pressures acting over a longer duration, bringing about a more serious change in the blood circulation.
The xe2x80x98finger blanchexe2x80x99 test is problematic in that it requires much subjective judgment on the part of the nurse or clinician, and the variances of skin pigmentation and condition make it difficult to observe changes over time. An existing laboratory based technique has been described to monitor and characterise skin redness using tissue reflectance spectroscopy (TRS). A fibre optic probe is used to deliver light and detect back-scattered light from the superficial dermis. The back-scattered light can be analysed for spectral components that have interacted with the blood. Using appropriate analysis algorithms it is possible to determine indices of blood content and oxygenation for the superficial skin vasculature.
Although these procedures allow precise and reliable measurements, their current use is limited to static measurement of quantities related to blood content in the skin. This does not necessarily relate to the seriousness of damage, which is established using manual methods by expelling blood from an area and observing the speed of re-colouration using the human eye. This dynamic measurement of the skin""s response to blanching, is a more accurate indication of the degree of damage caused to the microcirculatory system, because the damaged area contains a higher degree of extravascular blood than an undamaged area.
Response to blanching has been identified as having value both in identifying erythema, and in differentiating between different types of erythema: non-blanching erythema exhibits a fast recovery of blood content after blanching, whereas reactive hyperemia (blanching erythema) exhibits a slow response.
The apparatus and method of the present invention may also be applied to dermal and/or sub-dermal tissues, or any internal or external tissue surface whether of human or other mammals and whether natural or created artificially by way or surgery or injury.
An object of the present invention is to provide a rapid and non-invasive diagnostic apparatus and method for assessing and differentiating damage to tissue microcirculation, for instance non-blanching and blanching erythema of the skin, by the measurement of blood content and the response of tissue to blanching. The method of analysis makes the determination of this damage insensitive to any tissue pigmentation which may be present. The method and apparatus of the invention overcomes the limitation of current human eye analysis by manipulating the reflectance data to subtract the effect of skin pigment (melanin).
A further object of the present invention is to predict the susceptibility of undamaged tissue to ulceration should adverse circumstances arise and also to predict the likelihood of further deterioration in skin condition in patients exhibiting blanching erythema (based on a library of information derived in extensive clinical trials)
Additionally, the present invention can be used to differentiate bruised or necrotic dermal or sub-dermal tissue from healthy or erythemous tissue by analysing both the level of deoxygenated or oxygenated blood in an area together with the dynamic response using a specially constructed algorithm developed on a neural network system. The apparatus and method of the present invention forces the blood from an area of tissue and monitors the rate of return of the blood back into the area. High blood content is an indication of inflamed tissue (erythema), but the rate at which blood flows back following expulsion is a more powerful indicator of the state of tissue damage, because static colour data alone does not indicate the extent of infarction and obliteration of the microcirculation and therefore the likelihood of the skin breaking down further under continued pressure.
The same technique can also be used on non-damaged tissue to assess the likelihood of the development of pressure ulcers. The blanching response of suspect tissue may be compared with information derived during clinical trials and characterised using neural network techniques. Thus, xe2x80x9chigh riskxe2x80x9d tissue can be identified, and it is possible to take the appropriate steps to prevent further damage, for instance pressure ulcers.
For patients assessed to be at high risk according to the apparatus and methods herein described, that a pressure ulcer may be forming, the patient could be moved to a support surface providing pressure reduction or pressure relief on the body. The patient can be monitored more closely (and re-assessed using the instrument and methods described) to make sure that pressure on particular vulnerable areas is avoided.
Therefore, the present invention can be used to reduce the development of pressure ulcers, as well as the early detection of any incipient pressure ulcers or the susceptibility of the tissue to microcirculatory damage.
According to one aspect of the present invention, an apparatus is provided for evaluating microcirculatory status or damage adjacent the tissue surface comprising a means for applying a mechanical stimulus to an area of the tissue surface, means for delivering light to said area of tissue and one or more sensors for detecting the light recovered from the tissue, a processor means connected to the sensor(s) and delivery means for determining the attenuation of different wavelengths of light scattered within the tissue to provide an indication of the state of the microcirculation of said area of tissue. The blood content value against time is determined to provide an indication of the quantity and mobility of extra-vascular blood in said area of tissue
Preferably, the means for applying a mechanical stimulus comprises applying a pressure and more preferably comprises a housing having a blanching edge to apply a sliding force on the body surface to provide a blanching effect. Advantageously, the sliding blanche has the capacity to produce a very even and complete blanche, without the requirement for exact control of the applied pressure.
Preferably, the means for delivering light is a transmitter which may be located remote from the housing or optionally may be located within the housing for a more compact diagnostic apparatus. Preferably the sensor(s) may be located adjacent the edge within the housing for more accurate measurements.
It will be understood that light can include the range of electromagnetic radiation wavelength comprising both the visible and near visible spectrum. Also the choice of transmitter wavelength or frequency distribution, receiver sensitivity, relative transmitter and receiver geometry, and interpretation of the variation of scattering attenuation of light within the tissue surface and sub-surface layers with time before, during and after mechanical stimulus information of value for diagnosing status risk and predicting progression of other clinical conditions may be produced.
These conditions include diabetes, vascular insufficiency, leg ulcers as well as monitoring status of tissue flaps and skin grafts.
Preferably, the sliding action of the edge necessary to blanche may be powered to avoid inconsistencies that may arise during manual repetitive blanching. Preferably, the blanching edge may have rotary or linear movement, and there may comprise relative motion between the blanching edge and the receiver(s). Preferably, the movement of the blanching edge is at a controlled rate to give a known time interval between the blanching edge and the sensor(s). Preferably, the blanching edge may be oblique relative to the direction of motion, to displace blood laterally.
Preferably, the processor means also measures the level and ratio of oxygenated to deoxygenated blood in an area, which allows identification of bruised or necrotic skin areas as distinct from erythema by analysing the time variation of light attenuation data during the blanching process.
According to another aspect of the present invention there is provided a method for evaluating microcirculatory damage adjacent to the tissue surface comprising the steps of applying a pressure to an area of skin surface, exposing said area to light, measuring the attenuation of different wavelengths of the light scattered within the tissue and collected by the sensor or more sensor(s) and computing blood content against time upon removal of the application of pressure.
The shape of the blood recovery signal indicates the state of microcirculation of the tissue using a set of rules derived from clinical trials.
Preferably, the pressure applied is a sliding force on the surface of the tissue. This method of applying pressure (blanching) has the advantage of being in the main a surface effect, causing little perturbation of the deeper structures.